IT would be easier to demolish and rebuild the Sydney Harbour Bridge than restore farm land that has been sold for mining, Barnaby Joyce says.

Australian Greens leader Bob Brown said yesterday he'd like to see laws bought in that make it harder for foreign companies to buy Australian farms.

Nationals Senate leader Barnaby Joyce agreed the best quality, productive farming land should be protected from being purchased by local or foreign interests for mining.

"Prime agricultural land is irreplaceable," he said.

"It would make more sense if there was a coal mine under the Sydney Harbour Bridge or Sydney Opera House to dig them up and mine coal there.

"We can always build another Opera House or build another Harbour Bridge if we really needed to."

Senator Joyce said he didn't care whether Australian or overseas companies bought farming land, so long as it was left for agricultural purposes.